This invention relates to compositions useful in and methods for cleaning, solvating and/or removing plastic resins and polymers from manufactured articles or manufacturing equipment, such as in the production of optical lenses. More particularly, the invention relates to solvent and solvent mixtures used to remove residues and methods of removing residues of plastic lens resins and polymers from materials that come in contact with the polymers, such as, but not limited to, lenses, molds, holders, racks, tools, and equipment used in the process of manufacturing organic lenses.
In recent years, plastic lenses have seen greater utility in eyeglass and camera lenses as well as in optical devices since they are lighter, dyeable, and more durable than lenses made from inorganic components. Original work focused on developing transparent plastic resins and polymers that possessed these better characteristics and had a refractive index similar to optical glass, which was approximately 1.52. A popular resin discovered for this use, and widely used commercially today, was a material obtained by subjecting diethylene glycol bisallyl carbonate (DEGBAC) (PPG Industries, Inc. Trademark "CR-39") to radical polymerization. This resin had various positive attributes of impact resistance, light weight, dyeability, and good machinability in cutting, grinding and polishing processes. The resin was found to have a refractive index of 1.50, which was lower than the refractive index for inorganic lenses, around 1.52.
To achieve optical equivalence to the inorganic glass lenses, it was necessary to increase the central and peripheral thickness along with the curvature of the lens. This increased thickness was undesired among users of optical lenses despite the obvious positive benefits of the organic resin lens. Therefore, newer resins and polymeric materials have and will be developed containing higher refractive indexes that will result in thinner and lighter lenses.
As a method for increasing the refractive index of plastic lenses, there are known methods comprising copolymerizing a monomer mixture by adding to a conventional monomer another monomer, which imparts a higher refractive index to the resulting polymer. The higher refractive index polymer and plastic lens obtained is required to not only have a high refractive index (&gt;1.49), but also exhibit good physical, mechanical and chemical properties as an optical lens. The art of manufacture of optical lenses from plastics involves the use of a number of polymers and copolymers of acrylates, methacrylates, methyl methacrylates, polycarbonates, phthalates, isocyanates, polyethers, urethanes and other monomer structures, that are well known and documented. Recent monomer art has included the use of a halogen molecule such as chlorine or bromine which will contribute to increasing the refractive index.
The lens and polymer industry continues to evolve as work continues on developing higher refractive index materials. Recent work has involved the use of sulfur as a part of the polymer. Adding sulfur to the polymer matrix greatly increases the refractive index of the polymer in addition to maintaining the desirable physical and optical characteristics. The addition of sulfur also increases the chemical resistance of the polymer making it more difficult to clean the apparatus used to manufacture the optical lens.
The method of producing a plastic lens is well documented. The lens is produced by a method in which a monomer mixture is cast into a casting mold formed of a glass, metal or plastic mold piece and a gasket made from an elastomer (typically ethylene-vinyl acetate copolymer) or metal. The polymer may contain an additive, which aids in initiating, controlling and polymerizing the monomers. The mold is then heated to a predetermined temperature for a predetermined period of time, and may or may not be irradiated by ultraviolet light, for instance, or subject to chemical treatments that assist in initiating or controlling the polymerization of the plastic lens in a desirable manner. The process continues for a predetermined period of time until the desired level of polymerization is achieved. The lens is then usually taken out of the mold by separating the mold pieces and gaskets and then subjected to further processing.
The mold pieces and gaskets are usually very expensive items that require cleaning prior to reuse. Often the mold pieces will be contaminated with polymer which has overflowed to the external sides of the mold, thereby requiring cleaning. In addition this overflowed polymer will be found on the holders, racks, tooling, and any other apparatus or equipment used in the manufacturing process that comes in contact with the polymer. Because the design of the optical polymer attempts to ensure a lens product with tough physical characteristics and chemical resistance, any overflowed polymer will likewise also display these characteristics. Therefore, the removal of the overflowed material from equipment is very difficult and can be very costly if the cleaning technique used damages the tooling or equipment.
Current art employs a number of methods to remove the polymer, which fall into three general methods. The first method is mechanical, where the polymer is removed from desired equipment, tooling, and molds by physical means of scraping and sandblasting. This method has drawbacks in that it is labor intensive, messy, time consuming, and many times can damage the delicate molds and equipment. The second method is thermal, in which the polymer is burned off in ovens or by heated media such as sand. This method is undesirable because of the cost of energy, the volatile organic compounds it produces, and the potential for fire. In addition, the elevated temperature required to clean some of the parts may physically affect the part and render them useless. The third method is chemical in which the molds, tooling, and/or equipment is contacted with a chemical solution that allows the polymer to be removed. This method is desirable since it is usually more cost effective in labor and time than the other two methods.
Chemical cleaning methods for removal of undesired or overflowed polymer falls into the use of strong inorganic acids or alkali. Most commonly used in the art are strong inorganic acids, such as sulfuric, nitric, or hydrochloric acid. The oxidizing action of these acids is most effective at elevated temperatures and they are, therefore, used mainly at temperatures in excess of 140.degree. F. (60.degree. C.) in order to remove most of the undesired polymers. The drawback of the use of these acids is that they are hazardous materials, and can be very aggressive on most molds and equipment, thereby reducing the useful life.
In most instances, special equipment, handling, and special rooms are required to operate the cleaning process. The use of alkali, such as alkali metal hydroxides such as sodium and potassium hydroxide, have also been found in the art. Like strong acids, these materials will have similar limitations and drawbacks, and seem likewise to only be effective in high concentrations at high temperatures. In high concentrations, these materials have a negative impact on glass molds and can be costly in reducing the useful life of the mold. U.S. Pat. No. 5,130,393 discusses the use of a combination of methylene chloride and strong alkali for cleaning molds and also for assisting in releasing the lens from the mold. No reference was made to the conditions and/or concentrations used in cleaning, nor was any mention made as to the effectiveness with polymers that contain sulfur and or halogens.